Time/Voltage Characteristics - ABB REU 523 Technical Reference Manual

REU 523
4.1.3.5.
16
Combined Overvoltage and Undervoltage Relay
Technical Reference Manual
The voltage relay measuring the positive-phase-sequence voltage complements
other methods, based on frequency relay and overcurrent relay, for disconnecting a
smaller power plant.
The positive-phase-sequence function can also be used instead of the conventional
high-set three-phase undervoltage protection based on phase-to-phase voltages. For
example, this kind of undervoltage protection function can be used for disconnecting
motors in case of voltage interruption so that the motors are prevented from starting
simultaneously when the voltage becomes available again.
The relay has to be set to three-phase use, not to single-phase use, when the positive-
phase-sequence operation criteria is selected.

Time/voltage characteristics

At the IDMT characteristic, the operate time of the stage is a function of the voltage:
the greater the deviation from the setting value, the shorter the operate time. Three
time/voltage curve groups called A, B and C are available.
The overvoltage and undervoltage units can be given a definite-time or an inverse
definite minimum time operation characteristic. The settings of switches SGF4/1...2
determine the operation mode of stage U>, SGF4/3...4 that of stage U>>, switch
SGF4/5 determines the operation mode of stage U< and SGF4/6 that of stage U<<.
Refer to section "Settings" beginning from page 19.
Recording of the operate time does not start until the deviation between the
measured voltage and the setting value is 6 %. The operate time accuracy stated in
the technical data applies when the deviation is 10 % or greater.
Characteristics for overvoltage stages
The IDMT characteristic curve groups A and B are designed for overvoltage stages
U> and U>>. The stages U> and U>> can be configured to use different
characteristics. The relationship between time and voltage at inverse-time
characteristic can be expressed as follows:
×
k a
t s [ ]
= ------------------------------------------------- -

U U> –
×
b ----------------- - 0.5 –

U>
where
t = operate time [s]
k = time multiplier k> or k>>
U = measured voltage [V]
U> = set start voltage [V] for U> or U>>
a = constant 480
b = constant 32
c = constant 0.035
p = constant (see table 4.1.3.5-1)
+ c
p


1MRS 750942-MUM
ABB Automation